Like plants and algae, cyanobacteria obtain energy from photosynthesis, utilizing energy from sunlight and electrons from water to reduce carbon dioxide (CO2) and thereby ‘fix’ carbon into cell biomass. This photosynthetically-fixed carbon can then be used to make metabolites, such as carbohydrates, proteins, and fatty acids that are ultimately distributed to heterotrophic organisms. Besides their role as primary carbon fixation organisms, cyanobacteria can also be altered to produce useful products. For example, Synechococcus elongatus PCC 7942 has been engineered to produce isobutyraldehyde and butanol; Synechocystis sp. PCC 6803 has been modified produce ethanol and isoprene.
Cyanobacteria excel at carbon fixation, thanks to their complex carbon concentrating mechanism (ccm), which is comprised of bicarbonate pumps, carbon dioxide-uptake systems and the carboxysome. The carboxysome is an approximate 300 MDa compartment essential for carbon concentration, as it enhances carbon fixation by sequestering ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and carbonic anhydrase (CA) within a protein shell. In the carboxysome lumen, bicarbonate is converted into carbon dioxide by carbonic anhydrase. Such conversion increases the proportion of carbon dioxide to oxygen in the vicinity of Rubisco, which favors Rubisco's carboxylase activity, while the shell limits the loss of carbon dioxide into the bulk cytosol (Cai et. al, 2009).
Researchers have explored ways to express the β-carboxysome shell and cyanobacterial form 1B Rubisco in chloroplasts (Lin et al., 2014b; Lin et al., 2014a). However, constructs have not been generated that can assemble the functional multi-protein metabolic carboxysome core.